The invention discloses a reflux passive micro-mixer based on a jet principle in a micro-electromechanical system. N jet and reflux structures are arranged in the central axis direction of a mixing channel from left to right, n is not less than 4, each set of the jet and reflux structures is formed by connecting a reducing pipe, a jet nozzle, feedback channels and a diverging pipe in series in sequence from left to right; each set of the jet and reflux structures comprises two feedback channels which are symmetrically arranged frontwards and backwards relative to a central axis, each feedback channel is of an annular structure which is formed by connecting a feedback channel inlet side channel, a feedback channel transition channel, a feedback channel outlet side channel and a mixing zone, the inlets of the two feedback channel inlet side channels and the outlets of the two feedback channel outlet side channels are connected and communicated to form the mixing zones together; a low-pressure zone is generated near the jet nozzle, fluids with low mixing efficiency are mixed to be returned near the jet nozzle through the feedback channels, and then entrained and mixed by jet flows to promote the two liquids to be completely mixed.